1. Field
The present invention relates generally to semiconductor fabrication and TFT LCD fabrication, and more particularly to immersion lithography.
2. Description of Related Art
Semiconductor fabrication processes and thin film transistor liquid crystal display (“TFT LCD”) fabrication processes generally include an optical lithography step where light passes through a mask that includes a pattern. The pattern is focused by a lens and is projected onto the surface of a semiconductor wafer or TFT LCD substrate that is coated with a thin layer of resist. The pattern may be a particular layer of the device that includes transistor structures, contacts, conductors, and so forth. As feature sizes of devices continue to decrease, the resolution of the lithography process becomes more important. The resolution of the lithography process, in particular the width of the conductors and spaces between the conductors, is proportional to the wavelength of light used for patterning, and inversely proportional to the numerical aperture of the lens. Ideally, the resolution of the lithography process is improved when the wavelength is small and the numerical aperture is large.
Immersion lithography is a technique in which lithographic exposure is performed with an immersion fluid introduced between the lens and the resist-coated wafer. Use of this technique may result in an increase in numerical aperture proportional to the refractive index of the immersion fluid. Typically, the immersion fluid is a layer of deionized ionized water, which may increase the numerical aperture at 193 nanometer incident electromagnetic radiation by a factor of up to 1.43 (the refractive index of water).
Further decrease in semiconductor device feature size may require improved resolution in the lithography process. Use of high refractive index immersion fluids and resists in the immersion lithography process may enable improved lithographic resolution without requiring the use of smaller wavelengths of light for patterning.